I’ve owned my Ruger GP100 for a little over a year, and I’ve only had the time/money to take it out to the range three or four times

I'm a big advocate of dry fire & dry fire drills, especially for someone in your situation. It makes the time you do spend at the range much more useful.

You can use dry fire to improve your sight picture & trigger control, in which you're establishing a good sight picture, and actually pulling the trigger (not on a live round, of course) while watching the front sight and not disturbing the sight picture.

You can also do dry fire drills where the main focus is on gun handling skills, e.g. drawing from a holster, reloading, moving, etc. There are a metric ton of dry fire drills online. For starters, Google Ben Stoeger's 15 minute dry fire drill.

As a new-ish shooter, though, I recommend focusing on just practicing the fundamentals while at the range (sight picture & trigger control) than on gun handling for the time being. Getting a firm foundation in the fundamentals now will pay big dividends later. Or, stated differently, blow off the fundamentals now, it will always be problem later.

At the range, you can put up standard bullseye targets, shoot 5 rounds groups and track your score (be sure to practice that double action trigger, as well ) There are a bunch of links to printable targets. I pasted one below. At the range, I'd work on your basics until you're able to consistently (in single and double action) shoot 44-45 out of 50 on a distance-appropriate target (e.g. an NRA B-2 @ 50 ft).

Until you've gained some proficiency with the fundamentals, I'd hold off on any type of fancy or speedy drill at the range. In the meantime, in addition to dry fire practicing your trigger control, you can also be practicing your gun handling skills with those Stoeger drills, among others.